1. FIELD OF THE INVENTION
This invention relates to a fixture for introducing in a single edge of a beam-shaped specimen of a rigid brittle material such as ceramic material a prescribed pre-crack necessary for the test to be performed in evaluation of fracture toughness of the material.
2. DESCRIPTION OF THE PRIOR ART
Preparatory to design and use of a structure or a structural part, the various properties of the material used therefor must be precisely comprehended. The value of fracture toughness proves to be the most important of all of the characteristic values and is determined in the test for fracture toughness. For metallic materials which have demonstrated their attributes through long use, the method for introducing a fatigue pre-crack in a specimen prepared for the test of fracture toughness has been prescribed in ASTM E399 A2 for the purpose of pemitting acquisition of the value of fracture toughness appropriate from the viewpoint of fracture mechanics. To be specific, a pre-crack fulfilling such a necessary condition that the radius of curvature at the crack front should be such as to be regarded as substantially infinitesimal must be introduced by causing growth of a fatigue crack under the restricted condition that the stress intensity factor, K.sub.I should be not more than 60% of the fracture toughness, K.sub.IC of the material under test.
In such rigid and brittle materials as bearing steel, hard tool alloys, and ceramics, however, it is difficult to effect the controlled introduction of a prescribed fatigue pre-crack because the extension of a fatigue crack occurs at a notably high speed or, conversely, it takes an extremely long time before the pre-crack extends to a prescribed length because the extension of the fatigue crack proceeds very slowly. In any event, the introduction of a fatigue pre-crack in a rigid brittle material has proved to lack practicability in industrial use.
To solve the difficulty, a method has been recently devised and put to use which introduces a brittle pre-crack in a specimen from such materials as used for bearing steel and hard tool alloys, for example (Literature 1-S. Nunomura and S. Jitsukawa; Tetsu-to-Hagane, 64(1978), S853, Literature 2-T. Sadahiro; Journal of Japan Institute of Metals, 45(1981), p. 291, and Literature 3-R. Warren and B. Johannesson, Powder Metallurgy, 27(1984) p. 25). This method comprises mounting on an anvil possessing a groove at the center a beam-shaped specimen having an impression or a shallow notch formed in advance therein as illustrated in FIG. 1(d) such that the specimen perpendicularly intersects the groove as illustrated in FIG. 1(a), superposing on this specimen a pusher-possessing a flat lower surface, applying a compressive load on the specimen in the direction of the width (W) of the specimen thereby causing the free surface of the specimen above the groove of the anvil to protrude slightly into the groove and, by virtue of the protrusion, causing the surface portion of the aforementioned free surface of the specimen to generate tensile stress and induce a brittle crack extending from the aforementioned impression or notch. By this method, introduction of a brittle pre-crack of a prescribed length "a" is attained because the extension of the crack in the direction of the width (W) of the specimen ceases as the tensile stress rapidly dwindles with the growth of the crack. The brittle pre-crack which is introduced by this method has an extremely sharp front. In this respect, this method produces a pre-crack appropriate from the viewpoint of fracture mechanics. The method for the introduction of a brittle pre-crack described above is excellent in the sense that the introduction of the pre-crack is attained very quickly and conveniently as compared with the introduction of a fatigue pre-crack.
As concerns the calculation of the fracture toughness, K.sub.IC this value K.sub.IC is obtained by using actual test data in the computation of a mathematical formula which is derived from numerical analysis based on fracture mechanics. This analysis entails an indispensable precondition that the actually introduced pre-crack should substantially equal the hypothetical pre-crack in size and shape. For this reason, ASTM E399, besides the aforementioned provisions of A2 setting forth the conditions of stress in the introduction of fatigue pre-crack, covers provisions on the critical values such as of the inclination of extension of pre-crack and the deflection of course of extension, for example, in Paragraph 8.2 concerning the size and shape of the introduced fatigue pre-crack. Specifically, it is specified that the difference, .DELTA.a, of pre-crack length between the two lateral surfaces illustrated in FIG. 1(c) should be within 10% of the average value, a, of the pre-crack length on the fractured surface of the specimen and that the deflection of the plane direction of the extension of the pre-crack illustrated in FIG. 1(c) should be within 10.degree. from the plane orthogonal to the direction of the length of the specimen. (The former value will be referred to hereinafter as the inclination, .DELTA.a/a, of the extension of the crack and the latter value as the deflection, .DELTA.L/B, of the course of extension. The conditions, .DELTA.a/a.ltoreq.0.10 and .DELTA.L/B.ltoreq.tan 10.degree.=0.176, are indicated in ASTM E399, paragraph 8.2.)
The method for the introduction of a brittle precrack described above accords with the provisions of ASTM E399 in the sense that the radius of curvature of the pre-crack front can be regarded as a substantial infinitesimal amount. Since the extension and the cessation of the brittle crack both occur instantaneously, however, it is difficult to control the inclination of the extension of pre-crack and the deflection of course of extension while the growth of the pre-crack is in progress. This method, therefore, has a disadvantage in that it does not always fulfil the aforementioned provisions easily. Particularly in the case of ceramics, since the blank materials and the finished products are mostly in small sizes, the test is inevitably required to be performed on specimens of small dimensions. Moreover, the ceramics by nature possess very low toughness. The size and shape of the brittle pre-cracks to be introduced by this method, therefore, are conspicuously affected by numerous delicate factors such as, for example, deflection of the specimen from the position perpendicularly intersecting the central groove in the anvil, dispersion in the length in which the anvil supports the specimen, roughness and flatness of the lower surface of the pusher, rigidity of the pushher and the anvil, and deflection of mutual disposition of the pusher, the specimen, and the anvil.